docs: Map stable RTD version to VERSION
docs: Map stable RTD version to VERSION

When readthedocs requests to build the "stable" build, configure the
links into GitHub to search for the release version specified in the
root of the repository. This fixes the links for examples in the Cilium
github repository for the stable version of the docs, such as those used
in the kubernetes getting started guide.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@covalent.io>

tklauser/cilium,scanf/cilium,eloycoto/cilium,tklauser/cilium,cilium-team/cilium,tgraf/cilium,tgraf/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,eloycoto/cilium,eloycoto/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,scanf/cilium,tgraf/cilium,eloycoto/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,eloycoto/cilium,eloycoto/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,scanf/cilium,michi-covalent/cilium,scanf/cilium,tgraf/cilium,scanf/cilium,cilium-team/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium